This invention relates to microspheric bodies derived from brain cells. This invention also relates to the purification of these microspheric bodies to homogeneity and to their use in screening proposed therapeutic measures for effectiveness in impeding amyloid formation and disease progression in human brain affected by Alzheimer's disease and related conditions. More specifically, the present invention is directed to dense microspheres obtained in purified form from brain cells, to the preparing of the dense microspheres in a usable form, and to methods for using them in the identification of therapies for treating Alzheimer's disease and related conditions associated with the formation of amyloid fibrils in the brain.
Alzheimer's disease is an incurable brain disease affecting middle aged and elderly people throughout the world. According to most recent estimates, it is the 4th or 5th leading cause of death among North Americans, and is responsible for inestimable personal and social tragedy, loss of productivity, and custodial burden to society. There is presently no widely-accepted effective treatment for Alzheimer's disease.
The principal symptom (manifestation) of Alzheimer's disease is the loss of higher mental faculties, typified by the loss of memory and behavior referred to as "dementia." Dementia is a symptom complex or syndrome which can be seen in many brain diseases other than Alzheimer's disease, such as stroke, encephalitis and metabolic diseases. Since memory loss and dementia are relatively nonspecific symptoms, a certain and specific definition of Alzheimer's disease is based on the characteristic microscopic state of the brain, described initially by Marinesco, Alzheimer and others [see Alzheimer, A., Allgemeine Zeitschrift fur Psychiatrie 64: 146-148, (1907); Marinesco, G., Comptes Rendus des Seances de la Socieete de Biologie et ses Filiales 70: 606-608 (1911)].
The particular microscopic feature that is a universally accepted indicator of Alzheimer's disease, and that separates Alzheimer's disease from other causes of dementia, is the accumulation of large numbers of brain lesions referred to as senile plaques and neurofibrillary tangles. These lesions (microscopical areas of abnormal brain tissue), when found in suitable quantity in a brain sample, are the definitive criteria for the diagnosis of Alzheimer's disease.
The clinical diagnosis of Alzheimer's disease is often a difficult and imperfect task which generally relies initially on ruling out other treatable or clinically definable causes of dementia. In the appropriate clinical context, if the latter causes cannot be proven, Alzheimer's disease is often diagnosed antemortem, by exclusion, as the most probable diagnosis. Many indirect methods of diagnosis at present are being proposed and tested [see Conference Report, Khachaturian, Z. Arch. Neurol. 42: 1097-105 (1985)]; but the only certain and acceptable method for diagnosing Alzheimer's disease is by tissue microscopic histological study of a brain biopsy or necropsy sample, in which the above-mentioned sine qua non lesions are recognized by a certified specialist with adequate expertise.
Senile plaques in large quantities are essentially found only in the Alzheimer group of diseases; in contrast, neurofibrillary tangles are nonspecific, found in at least ten other neurological diseases [see Corsellis, J.A.N., GREENFIELD's EUROPATHOLOGY 951-1025 (4th ed. 1984) (Edward Arnold, London)]. Individual senile plaques have roughly 1000X the volume of individual neurofibrillary tangles. True measurements of total brain senile plaque and neurofibrillary tangle content are not available, but on the above basis it is likely that the volume of abnormal brain tissue occupied by senile plaques is many hundreds of times that of neurofibrillary tangles. The essential feature of the senile plaque is the presence of amyloid fibrils ("amyloid"), which are a congophilic red-green birefringent microfibrillar material (Corsellis, loc. cit.).
The utilization of materials found in human brain (normal or affected by Alzheimer's disease) that are not initially amyloid and transforming them into amyloid has not been documented. Thus an experimental system, derived from human materials, characterized by the sine qua non feature of human Alzheimer's disease has not been documented. A fundamental approach to treating a disease is to reproduce the disease experimentally in a nonhuman context and to test potential treatments for their effect on the experimental disease. Because the presence of amyloid provides a definitive indication of senile-plaque formation, most specialists agree that reproduction of amyloid fibrils experimentally from precursor materials which are extracted, activated, or otherwise derived from the human brain would constitute the best available evidence linking an agent or precursor to the progression of Alzheimer's disease. Despite much investigation into this question during the past fifty years, the fundamental step of reproducing amyloid experimentally from materials derived solely from human brain tissue has not been documented.
A microscopic structure referred to as the dense microsphere is known to exist in normal brain and in brain affected by Alzheimer's disease brain (Averback, Acta Neuropathol. 61: 148-52 (1983); results confirmed by Hara, M., J. Neuropath. Exp. Neurol. 1986). Some specialists believe that dense microspheres ("DMS") may be connected somehow to the formation of amyloid senile plaque, but this hypothessis has not been proven. Evidence for the existence of DMS comes from microscopic histological section studies of fixed whole brain tissue, where the dense microspheres are observed as randomly dispersed, highly infrequent structures occupying an estimated 10.sup.-9 or less of total brain volume, at a unit frequency roughly estimated at 10.sup.-16 or less, relative to other definable brain structures such as mitochondria. But the extraction, purification and characterization of isolated samples, and the use of tangible DMS material to any advantage, have not been documented. Without these developments, DMS are structures of unproven function, unknown significance or usefulness, are not in tangible form and are not readily available.